Limit `futures` dependency to `Stream` via feature flag (#1774)

In an effort to reach API stability, the `tokio` crate is shedding its
_public_ dependencies on crates that are either a) do not provide a
stable (1.0+) release with longevity guarantees or b) match the `tokio`
release cadence. Of course, implementing `std` traits fits the
requirements.

The on exception, for now, is the `Stream` trait found in `futures_core`.
It is expected that this trait will not change much and be moved into `std.
Since Tokio is not yet going reaching 1.0, I feel that it is acceptable to maintain
a dependency on this trait given how foundational it is.

Since the `Stream` implementation is optional, types that are logically
streams provide `async fn next_*` functions to obtain the next value.
Avoiding the `next()` name prevents fn conflicts with `StreamExt::next()`.

Additionally, some misc cleanup is also done:

- `tokio::io::io` -> `tokio::io::util`.
- `delay` -> `delay_until`.
- `Timeout::new` -> `timeout(...)`.
- `signal::ctrl_c()` returns a future instead of a stream.
- `{tcp,unix}::Incoming` is removed (due to lack of `Stream` trait).
- `time::Throttle` is removed (due to lack of `Stream` trait).
-  Fix: `mpsc::UnboundedSender::send(&self)` (no more conflict with `Sink` fns).

1 files changed, 2 insertions, 1 deletions

diff --git a/tokio/src/sync/barrier.rs b/tokio/src/sync/barrier.rs
index 1582120e..911e78fe 100644
--- a/tokio/src/sync/barrier.rs
+++ b/tokio/src/sync/barrier.rs
@@ -8,8 +8,9 @@ use std::sync::Mutex;
 /// # #[tokio::main]
 /// # async fn main() {
 /// use tokio::sync::Barrier;
+///
+/// use futures::future::join_all;
 /// use std::sync::Arc;
-/// use futures_util::future::join_all;
 ///
 /// let mut handles = Vec::with_capacity(10);
 /// let barrier = Arc::new(Barrier::new(10));